Platinum-catalyzed intramolecular hydroarylation of allenyl arenes: Efficient synthesis of 1,4-dihydronaphthalenes

Article abstract of DOI:10.1021/ol1007857

An efficient synthetic method of 1,4-dihydronaphthalenes having various alkyl groups on the 4-position and/or ethoxycarbonyl group on the 2-position through selective intramolecular Pt-catalyzed 6-endo cyclohydroarylation of ethyl 2-benzyl-2,3-alkadienoat

Full text of DOI:10.1021/ol1007857

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2570-2573
Platinum-Catalyzed Intramolecular
Hydroarylation of Allenyl Arenes:
Efficient Synthesis of
1,4-Dihydronaphthalenes
Juntae Mo and Phil Ho Lee*
National Research Laboratory for Catalytic Organic Reaction, Department of
Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon
National UniVersity, Chuncheon 200-701, Republic of Korea
phlee@kangwon.ac.kr
Received April 5, 2010
ABSTRACT
An efficient synthetic method of 1,4-dihydronaphthalenes having various alkyl groups on the 4-position and/or ethoxycarbonyl group on the
2-position through selective intramolecular Pt-catalyzed 6-endo cyclohydroarylation of ethyl 2-benzyl-2,3-alkadienoates was developed. The
present method could be further extended to two-fold Pt-catalyzed 6-endo and 7-endo cyclizations.
Since 1,4-dihydronaphthalene is an important structural
motif frequently found in natural products and has been
used as an essential skeleton in pharmaceutics, develop-
ment of efficient and versatile synthetic methods for 1,4-
dihydronaphthalene derivatives has been continuously
required,1 and many synthetic strategies for 1,4-dihy-
dronaphthalenes have been reported in the literature.2
Among them, Birch reduction has been widely used for
obtaining 1,4-dihydronaphthalene due to the availability
and stability of naphthalene derivatives.3 However, the
Birch reduction has some limitations. Only unfunction-
alized alkyl groups can be introduced during the dearo-
matization, and many functional groups are not tolerable
under the reaction conditions. Regioselectivity is often
poor, and over-reduction is another common problem.
Although recent modifications of the Birch reduction, such
as the use of silylated naphthalenes, expand its utility,4
regio- and stereocontrol are still difficult. Lautens has
reported that the aryne Diels-Alder reaction with acyclic
dienes is a useful method for the synthesis of function-
alized 1,4-dihydronaphthalenes.5 Recently, intramolecular
transition-metal-catalyzed hydroarylation of alkynes6 and
allenes7 has been intensively investigated, providing a
wide range of carbocyclic as well as heterocyclic com-
pounds. In addition, Widenhoefer reported intramolecular
(3) (a) Hook, J. M.; Mander, L. N. Nat. Prod. Rep. 1986, 3, 35. (b)
Rabideau, P. W.; Marcinow, Z. Org. React. 1992, 42, 1. (c) Rabideau, P. W.
Tetrahedron 1989, 45, 1579. (d) Rabideau, P. W.; Fronczek, F. R. Chem.
Commun. 1992, 79. (e) Birch, A. J. Pure Appl. Chem. 1996, 68, 553. (f)
Subba Rao, G. S. R. Pure Appl. Chem. 2003, 75, 1443.
(1) (a) Gozler, T.; Gozler, B.; Patra, A.; Leet, J. E.; Freyer, A. J.;
Shamma, M. Tetrahedron 1984, 40, 1145. (b) Castro, M. A.; Gordaliza,
M.; Del Corral, J. M. M.; Feliciano, A. S. Phytochemistry 1996, 41, 995.
(c) Ward, R. S. Nat. Prod. Rep. 1997, 14, 43. (d) Winemiller, M. D.;
Harman, W. D. J. Am. Chem. Soc. 1998, 120, 7835.
(4) (a) Rabideau, P. W.; Karrick, G. L. Tetrahedron Lett. 1987, 28, 2481.
(b) Marcinow, Z.; Clawson, D. K.; Rabideau, P. W. Tetrahedron 1989, 45,
5441.
(2) (a) Stephan, D.; Gorgues, A.; Coq, A. L. Tetrahedron Lett. 1984,
25, 5649. (b) Mander, L. N.; Morris, J. C. J. Org. Chem. 1997, 62, 7497.
(c) Kawahata, N. H.; Goodman, M. Tetrahedron Lett. 1999, 40, 2271.
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J. Am. Chem. Soc. 2005, 127, 15028, and references therein.
10.1021/ol1007857  2010 American Chemical Society
Published on Web 05/13/2010

Pt- or Au-catalyzed hydroarylation of alkenes and allenes
with indole derivatives.8 In this respect, we envisioned
that 1,4-dihydronaphthalene derivatives might be formed
by intramolecular transition-metal-catalyzed hydroaryla-
tion reaction of allenyl arenes. In this paper, we report
the direct formation of 1,4-dihydronaphthalenes having a
variety of alkyl groups on the 4-position and/or ethoxy-
carbonyl group on the 2-position through selective in-
tramolecular Pt-catalyzed hydroarylation of ethyl 2-benzyl-
2,3-alkadienoates in a 6-endo mode (Scheme 1). Also,
Scheme 2. Preparation of Ethyl 2-Benzyl- and
2-Phenethyl-2,3-alkadienoates
that 2.5 mol % of PtCl4/10 mol % of AgSbF6 was effective
for hydroarylation, yielding 2-ethoxycarbonyl-1,4-dihy-
dronaphthalene (2a) in 72% yield in ClCH2CH2Cl at 80 °C
for 2 h. Encouraged by this preliminary result, we further
examined several catalysts and silver cocatalysts. As shown
in Table 1, a better result was obtained with 2.5 mol % of
Scheme 1. Intramolecular Hydroarylation Catalyzed by Platinum
Table 1. Reaction Optimization
two-fold-selective intramolecular Pt-catalyzed hydroary-
lation in a 6-endo mode is described, producing tetrahy-
droanthracene and tetrahydrophenanthrene derivatives.
Furthermore, intramolecular Pt-catalyzed hydroarylation
in a selective 7-endo mode is reported.
time yielda
First, a variety of ethyl 2-benzyl- and 2-phenethyl-1,2-
alkadienoates were prepared by treatment of (carbethoxy-
benzylidene)triphenylphosphorane or (carbethoxypheneth-
ylidene)triphenylphosphorane with alkanoyl chloride having
an R-hydrogen in the presence of triethylamine in dichlo-
romethane (Scheme 2).2a,c,9
entry
catalyst
5 mol % of AuCl3
(h)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
12
12
0
0
0
0
0
5 mol % of AuCl3/15 mol % of AgOTf
5 mol % of Ph3PAuCl/5 mol % of AgOTf 12
5 mol % of PtCl4
5 mol % of PtCl2
2.5 mol % of PtCl4/10 mol % of AgSbF6
2.5 mol % of PtCl2/5 mol % of AgSbF6
5 mol % of AgSbF6
2.5 mol % of PtCl2/5 mol % of AgOTf
2.5 mol % of PtCl2/5 mol % of AgNTf
2.5 mol % of PtCl2/5 mol % of AgPF6
2.5 mol % of PtCl2/5 mol % of AgAsF6
2.5 mol % of PtCl2/5 mol % of AgBF4
18
6
2
2
6
2
2
6
6
6
To examine the feasibility of intramolecular hydroarylation
of ethyl 2-benzyl-2,3-butadienoate (1a), when the reaction
was carried out with several gold catalysts such as 5 mol %
of AuCl3, 5 mol % of AuCl3/15 mol % of AgOTf, and 5
mol % of Ph3PAuCl/5 mol % of AgOTf in DCE, the reaction
did not proceed. Although hydroarylation catalyzed by 5 mol
% of PtCl4 and 5 mol % of PtCl2 did not occur, we found
72 (14)b
89
0
93
88
0
2
0
0
a Isolated yield.; 0% yield means no reaction and then 1a was recovered.
(6) (a) Chatani, N.; Inoue, H.; Ikeda, T.; Murai, S. J. Org. Chem. 2000,
65, 4913. (b) Fu¨rstner, A.; Mamane, V. J. Org. Chem. 2002, 67, 6264. (c)
Nishizawa, M.; Takao, H.; Yadav, V. K.; Imagawa, H.; Sugihara, T. Org.
Lett. 2003, 5, 4563. (d) Fu¨rstner, A.; Mamane, V. Chem. Commun. 2003,
2112. (e) Reetz, M. T.; Sommer, K. Eur. J. Org. Chem. 2003, 3485. (f)
Mamane, V.; Hannen, P.; Fu¨rstner, A. Chem.sEur. J. 2004, 10, 4556. (g)
Nevado, C.; Echavarren, A. M. Synthesis 2005, 167. (h) Goj, L. A.; Gunnoe,
T. B. Curr. Org. Chem. 2005, 9, 671. (i) Soriano, E.; Marco-Contelles, J.
Organometallics 2006, 25, 4542. (j) Chernyak, N.; Gevorgyan, V. J. Am.
Chem. Soc. 2008, 130, 5636. (k) Menon, R. S.; Findlay, A. D.; Bissember,
A. C.; Banwell, M. G. J. Org. Chem. 2009, 74, 8901.
b Ethyl 2-naphthoate.
PtCl2/5 mol % of AgSbF6 (89%, 2 h, entry 7). The individual
use of 5 mol % of AgSbF6 without PtCl2 did not afford the
cyclized compound (entry 8). Among various silver cocata-
lysts examined, AgOTf turned out to be the most effective,
thus providing 2a selectively in a 6-endo mode in 93% yield
in ClCH2CH2Cl at 80 °C for 2 h (entry 9). AgNTf2 was
slightly less effective (88%, entry 10). It is noteworthy that
use of 5 mol % of AgPF6, AgAsF6, and AgBF4 as a cocatalyst
in the presence of 2.5 mol % of PtCl2 was totally ineffective
for intramolecular hydroarylation (entries 11-13). Interest-
ingly, the isomerized R,ꢀ-unsaturated ester 3 and five-
membered cyclic compound 4 in 5-exo mode were not
detected.
(7) (a) Watanabe, T.; Oishi, S.; Fujii, N.; Ohno, H. Org. Lett. 2007, 9,
4821. (b) Tarselli, M. A.; Gagne, M. R. J. Org. Chem. 2008, 73, 2439. (c)
Weber, D.; Tarselli, M. A.; Gagne, M. R. Angew. Chem., Int. Ed. 2009,
48, 5733. (d) Kong, W.; Cui, Y.; Chen, G.; Fu, C.; Ma, S. Org. Lett. 2009,
11, 1213.
(8) (a) Liu, C.; Han, X.; Wang, X.; Widenhoefer, R. A. J. Am. Chem.
Soc. 2004, 126, 3700. (b) Han, X.; Widenhoefer, R. A. Org. Lett. 2006, 8,
3801. (c) Zhang, Z.; Liu, C.; Kinder, R. E.; Han, X.; Qian, H.; Widenhoefer,
R. A. J. Am. Chem. Soc. 2006, 128, 9066. (d) Liu, C.; Widenhoefer, R. A.
Org. Lett. 2007, 9, 1935.
(9) (a) Ma, S.; Jiao, N.; Zhao, S.; Hou, H. J. Org. Chem. 2002, 67,
2837. (b) Zhu, X.-F.; Lan, J.; Kwon, O. J. Am. Chem. Soc. 2003, 125,
4716. (c) Shen, R.; Huang, X. J. Org. Chem. 2007, 72, 3961. (d) Tran,
Y. S.; Kwon, O. J. Am. Chem. Soc. 2007, 129, 12632. (e) Castellano, S.;
Fiji, H. D. G.; Kinderman, S. S.; Watanabe, M.; de Leon, P.; Tamanoi, F.;
Quenching of the reaction mixture with D2O after in-
tramolecular hydroarylation of 1a gave rise to the corre-
sponding deuterated adduct 2a in 96% yield with 10%
Kwon, O. J. Am. Chem. Soc. 2007, 129, 5843
.
Org. Lett., Vol. 12, No. 11, 2010
2571

Table 2. Pt-Catalyzed Cyclization of Allenyl Esters
d-incorporation (Scheme 3). Also, addition of 10% D2O as
a cosolvent to ClCH2CH2Cl provided 2a in 95% yield with
Scheme 3
.
Deuterium Incorporation in Pt-Catalyzed
Hydroarylation
34% d-incorporation, indicating that a vinylplatinum inter-
mediate might be formed.10
To demonstrate the efficiency and scope of the present
method, we applied this catalytic system to a wide range of
ethyl 2-benzyl-2,3-alkadienoates 1. Intramolecular hydroary-
lation in the presence of a methyl and ethyl group on the
terminal sp2 carbon of the allenyl group proceeded smoothly
to afford dihydronaphthalenes 2b and 2c in 90 and 88%
yields, respectively (Table 2, entries 2 and 3). In the case of
1d, 5 mol % of PtCl4/20 mol % of AgOTf was superior,
resulting in the formation of cyclic compound 2d in 87%
yield (entry 4). The presence of 2-methyl, 4-methyl, and 3,5-
dimethyl groups on the phenyl ring exhibited little effect on
either the reaction rate or product yield (entries 5-10).
However, allenes 1g and 1j having a phenyl group on the
terminal sp2 carbon of the allene group were slightly less
effective (76 and 82% yields, entries 7 and 10). Treatment
of 1k bearing a 3-methoxy group with 2.5 mol % of PtCl2/5
mol % of AgOTf led to the mixtures of isomeric 1,4-
dihydronaphthalenes, but the product (2kb) resulting from
the 2-position predominates (entry 11). Under the optimum
reaction conditions, naphthyl alkadienoates 1l and 1m were
smoothly converted to 1,4-dihydroanthracenes 2l and 2m in
92 and 88% yields, respectively (entries 12 and 13). It was
gratifying to obtain 1,4-dihydronaphthalenes 2n and 2o
possessing iodide and bromide groups in 74 and 95% yields,
respectively (entries 14 and 15). When ethyl 2,3-butadienoate
bearing both a methoxy and a bromide group 1p was
subjected to the reaction conditions, the corresponding
dihydronaphthalene 2p was obtained in 78% yield (entry 16).
Next, two-fold intramolecular hydroarylations were briefly
examined. When bisallenyl benzene 1q was subjected to the
standard condition, tetrahydroanthracene 2q and tetrahydro-
phenanthrene 2r were isolated in 52 and 32% yields,
a Using 5 mol % of PtCl4 and 20 mol % of AgOTf. b Using 5 mol %
of PtCl2 and 10 mol % of AgOTf. c Ratio of 2-ethoxycarbonyl-5-methoxy-
1,4-dihydronaphthalene (2ka) and 2-ethoxycarbonyl-7-methoxy-1,4-dihy-
dronaphthalene (2kb).
respectively (Scheme 4). The present method worked equally
well with 1r, producing 1,4,5,8-tetrahydroanthracene 2s in
82% yield.
Finally, when ethyl 2-phenethyl-2,3-butadienoate (1s) was
treated with Pt catalyst, seven-membered cyclic compound
2t was selectively obtained in 64% yield. Apparently,
intramolecular hydroarylation proceeded in a 7-endo mode
without contamination of the 6-exo mode (Scheme 5).
Although the mechanism of the present reaction has not
been established, a possible reaction mechanism for intramo-
(10) Nelsen, D. L.; Gabne, M. T. Organometallics 2009, 28, 950.
2572
Org. Lett., Vol. 12, No. 11, 2010

Scheme 4
.
Two-Fold Intramolecular Hydroarylation Catalyzed
by Platinum
Scheme 6. Plausible Reaction Mechanism
through Pt-catalyzed 6-endo cyclohydroarylation of ethyl
2-benzyl-2,3-alkadienoates. The present method could be
further extended to two-fold Pt-catalyzed 6-endo and 7-endo
cyclizations. This method would pave a new way to
synthetically valuable processes of a wide range of 1,4-
dihydronaphthalene, 1,4,5,8-tetrahydroanthracene, and tet-
rahydrophenanthrene derivatives.
Scheme 5
.
Pt-Catalyzed Intramolecular Hydroarylation in
7-endo Mode
Acknowledgment. This work was supported by the NRL
Program funded by the MEST and by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (2009-0087013). This work was sup-
ported by the second phase of the Brain Korea 21 Program
in 2009. Dr. Sung Hong Kim at the KBSI (Daegu) is thanked
for obtaining the MS data. The NMR data were obtained
from the central instrumental facility in Kangwon National
University.
lecular Pt-catalyzed hydroarylation is shown in Scheme 6.
The reaction would be initiated by activation of the allenyl
group by platinum catalyst and followed by 6-endo cycliza-
tion to afford zwitterionic intermediate vinylplatinum 6.
Aromatization to vinylplatinum 7 and subsequent protonation
would provide 1,4-dihydronaphthalene 2. Deuterium incor-
poration (34%) shown in Scheme 3 indicates that the
vinylplatinum 7 might be formed. The elucidation of the
detailed reaction mechanism must wait further study.
In summary, we developed an efficient synthetic method
of 1,4-dihydronaphthalenes having various alkyl groups on
the 4-position and/or ethoxycarbonyl group on the 2-position
Supporting Information Available: Experimental pro-
cedure and spectral data. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL1007857
Org. Lett., Vol. 12, No. 11, 2010
2573